He finished just eight votes ahead of former senator Rick Santorum in the Midwest state of Iowa.
Ron Paul came third, while Newt Gingrich and Michele Bachmann settled into a second tier of candidates.
Rick Perry indicated he was suspending his campaign after finishing fifth.
The caucus meetings were the first time voters had a say in the race to face Democratic President Barack Obama in November’s presidential election.

Tuesday’s contest launched months of caucuses and primary elections in 50 states, Washington DC and other territories, culminating in the Republican National Convention in August where the party nominee will be formally anointed.
Iowa was not expected to settle the contest – John McCain, the eventual Republican nominee in 2008, came fourth in the state’s caucuses that year – but it will help shape the race for the White House.

The BBC’s Mark Mardell says that in the end, this result has to be good for Mitt Romney, achieving it in a state he hadn’t bothered to fight until the last few weeks.
‘Game on!’
Hours after caucuses closed, Iowa party chairman Matt Strawn announced that Mr Romney had won by just eight of the 122,255 votes cast.
“Governor Mitt Romney received 30,015 votes and senator Rick Santorum received 30,007 votes. Congratulations to governor Mitt Romney, winner of the 2012 caucuses. Congratulations to senator Rick Santorum for a very close second-place finish,” Mr Strawn said.
Earlier in the evening, Mr Romney kept his sights firmly trained on Mr Obama rather than engaging his Republican rivals or claiming victory.
“The gap between his promises four years ago and his performance is as great as anything I’ve ever seen in my life,” he said, before crying: “On to New Hampshire!”
As early results came out pointing to a close race, Mr Santorum declared “Game on!” He praised his faith and his family in a speech which marked his own entry to the national spotlight.

Mr Paul, a Texas congressman, finished third and vowed to continue onto New Hampshire, which holds a primary election next week.
“This momentum is going to continue,” he told a jubilant crowd of supporters. “We will go on, we will raise the money.”

Negative advertising
Finishing fifth, Texas Governor Rick Perry said he was returning to his home state in order to “determine whether there is a path forward for myself in this race”.Former House of Representatives Speaker Newt Gingrich, who saw a brief lead evaporate under a barrage of negative advertising last month, pledged to remain in the race and challenge Mr Romney, “a Massachusetts moderate”.
“We are not going to go out and run nasty ads,” said Mr Gingrich, who finished fourth.
“But I do reserve the right to tell the truth. And if the truth seems negative that may be more of a comment on his record than it is about politics.”
Mrs Bachmann, a Minnesota congresswoman who won the Iowa straw poll last summer, finished sixth, and urged voters not to let the media anoint a Republican nominee based solely on the Iowa results.
Former Utah Governor Jon Huntsman was on the ballot, but did not campaign in Iowa.

Gary Dobson and David Norris were found guilty by an Old Bailey jury after a trial based on forensic evidence.
They will be sentenced as juveniles because they were under 18 at the time of the attack, which happened in south-east London in April 1993.Police say the investigation could be reopened if new evidence emerges.
Dobson, 36, and Norris, 35, can expect to receive sentences considerably shorter than would an adult convicted of the same crime under today’s laws.The BBC’s Home Affairs Correspondent Matt Prodger says they could serve minimum prison terms of around 12 years each.
Scientists found a tiny bloodstain on Dobson’s jacket that could only have come from Mr Lawrence. They also found a single hair belonging to the teenager on Norris’s jeans.
Acting Deputy Commissioner of the Metropolitan Police, Cressida Dick, who ordered the 2006 cold case review that led to the convictions, acknowledged that police believe there were five people involved in the murder, but there are currently no “live” lines of inquiry.
“If there was an opportunity to bring the other people who were involved in that night to justice, we would do so,” she said.
Give up others’
In a statement read by his lawyer outside the Old Bailey on Tuesday, Stephen’s father, Neville Lawrence, said the convictions were a moment of joy and relief – but he could not rest until all of those who killed his son were brought to justice. He described the investigation and preparation of the case as “faultless”.
He later told Channel 4 News: “I’m praying that these people now realise that they’ve been found out and say to themselves, ‘yes I did this awful deed, but I wasn’t alone in that action that night and there are other people also guilty of what I’ve done’ and name them.
“I hope before the sentence is passed, they will talk and give the rest of these people that killed my son up.”In an exclusive interview with the BBC’s Panorama, Stephen’s mother Doreen Lawrence said: “I don’t forgive the boys who killed Stephen. They don’t think they have done anything wrong.
“They took away Stephen’s life and there is nothing in their behaviour or anything to show they regret what their actions have done and the pain it has caused us as a family.”Prime suspects’
The original failed investigation into the murder led to the Metropolitan Police being branded as institutionally racist.
Stephen Lawrence was 18 when he was stabbed to death near a bus stop in Eltham, south east London, in April 1993.
Police identified five men who were later named in a damning public inquiry as the “prime suspects”.
By that time, there had already been a catalogue of police errors and two failed prosecutions, one brought by Stephen’s parents.

Iran nuclear crisis: Sanctions ‘beginning to bite’

The US has said threats by Iran to restrict Gulf shipping in the event of further sanctions shows international pressure is having an effect.

The State Department said sanctions on Tehran over its nuclear programme were starting to bite and that Iran was trying to create a distraction.

Iran has conducted 10 days of exercises near the strategically vital Strait of Hormuz, test-firing several missiles.

Its currency is at a record low, but it has denied sanctions are to blame.

The UN Security Council has already passed four rounds of sanctions against Iran for refusing to halt uranium enrichment.

Highly enriched uranium can be processed into nuclear weapons, but Iran denies Western charges that it is trying to develop them.

Tehran says its programme is peaceful – it needs nuclear technology to generate electricity to meet growing domestic demand.The US has also sanctioned dozens of Iranian government agencies, officials and businesses over the nuclear programme.

The government in Tehran has dismissed the latest measures announced in the wake of a critical IAEA report in November.

US President Barack Obama signed into law the US bill targeting Iran’s central bank on Saturday. It enters into force in six months’ time.

Since then, however, the Iranian national currency, the rial, has lost about 12% of its value – trading at about 17,200-18,000 rials to $1.

Earlier on Tuesday, French Foreign Minister Alain Juppe called for “stricter sanctions” and urged EU countries to follow the US in freezing Iranian central bank assets and imposing an embargo on oil exports.
‘Mock’ exercises

Speaking to journalists, the State Department’s Victoria Nuland said Tehran was feeling increasingly isolated because of the sanctions.

“Frankly we see these threats from Tehran as just increasing evidence that the international pressure is beginning to bite there and that they are feeling increasingly isolated and they are trying to divert the attention of their own public from the difficulties inside Iran, including the economic difficulties as a result of the sanctions,” she said.

Meanwhile Pentagon spokesman George Little responded to Iranian warnings to keep an aircraft carrier out of the Gulf, saying the Navy was operating within international law and had no plans to pull warships out of the region.Iran has been holding a series of naval exercises in the Gulf, and on Monday

said it had successfully test-fired a surface-to-sea Qader cruise missile, a shorter range Nasr and later, a surface-to-surface Nour missile.

Iran has conducted 10 days of exercises near the Strait of Hormuz, through which 20% of the world’s traded oil passes.

Tehran said on Monday that “mock” exercises on shutting the strait had been carried out, although there was no intention of closing it.

The BBC’s Iran correspondent James Reynolds says Iran is using the exercises to try to show that it owns the Gulf and has the military capability to defend against any threat to its dominance.

But, says our correspondent, few believe Iran would carry out its threat to shut the Strait of Hormuz as to do so would be considered too economically, politically and possibly militarily damaging for Tehran.